Keggin heteropolyacids

It is worth noting that differences in acid strength between anhydrous Keggin heteropolyacids (H3PW12O40 > H4SiWi2O40 > H3PM012O40 > H4SiMoi204o) measured by ammonia adsorption calorimetry did not correlate simply with the catalytic activity in the reaction of rapeseed oil transesterification with methanol and ethanol [102]. 

The acidity of the HPA was compared with several tungsten- and molybdenum-based materials such as bulk tungsten trioxide, monolayer supported tungsten species, nanoparticulate mesoporous tungsten-zirconium oxide, and Keggin heteropolyacids. 

In this context, much effort has also been invested in controlling the nuclearity of the catalyst ensemble through the selection of its precursor. One area in which considerable progress has been made involves the adsorption of polynuclear clusters onto supports [33]. Examples involving the immobilization of small, preformed polynuclear clusters on supports are the reactions of carbonyl clusters of the late metals [16, 34], the binding of polyoxometalates (POMs) and their neutral alkoxy analogues [35] and heteropolyacids such as the Keggin cluster [36, 37]. 

Herve et al. (57) investigated the thermal changes of structures by means of XRD and TG-DTA for Keggin-type heteropolyacids and proposed Scheme 2. Infrared spectroscopy of H4PMo, VO40 showed the release of vanadium atoms to form H3PM012O40 and vanadium phosphate species (55). Exposure to water vapor induces the decomposition of the latter (indicated by the disappearance of a band at ca. 1037-1030 cm -1) (58). 

Figure 23 shows a correlation between the catalytic activity and the Hammett acidity function (H0) of Keggin-type heteropolyacids in CH3CN. The catalytic activity increases with the acid strength (63). 

Table XVII is a comparison of the catalytic activities for liquid-phase MTBE synthesis from isobutylene and methanol (179). The catalyst structure and composition have a strong effect on the activity. The highest activity per proton was obtained with a Dawson-type heteropolyacid, H6P2W 8062, although the acid strength of H WigO is lower than that of the Keggin-type H3PW12O40 (Section HI). Water added to the mixture has little effect on the reaction rate at water concentrations less that 2 wt%, but at 5 wt% the rate is less by a factor of 2.5. At the same time the selectivity is less due to the formation of (erf-butyl alcohol.

It has been pointed out that these pillared intercalates are intrinsically difficult to synthesize in highly crystalline form because the layered hosts are basic, whereas most heteropolyacids are acidic and tend to decompose. Narita et al. (392) tried direct synthesis of a heteropolyanion-pillared layered double hydroxide by a coprecipitation reaction of Zn2+ and A1J+ ions in the presence of a moderately acidic lacunary Keggin anion, a-SiWn039 XRD of the product showed a basal spacing of 14.6 A, which corresponds to a gallery height of 9.9 A. The surface area was found to be 97 m2 g, which is three times that of the layered double hydroxide. 

Figure 3.11 FTIR skeletal spectra of a-Zr(HPOi)2 H2O, CdMo04 (scheelite type) and the Keggin-type heteropolyacid salt K3PM012O40.

Keggin-type heteropoly compounds have attractive and important characteristics in terms of catalysis. They consist of heteropolyanions and counter-cations such as H, Cs or NHT When the counter-cations are protons, they are called heteropolyacids (HPA). An important characteristic of HPAs, such as 12-tungstophos-phoric acid (H3PW12O40), is the presence of very strong Bronsted acid sites. But the characteristics of HPAs strongly depend on temperature and relative humidity. When they are used in heterogeneous catalysis, it is often necessary to support them on high-surface-area oxides or activated carbons, in order to increase the surface contact with the reactants. 

The number and strength of the acid centers of tungstic heteropolyacids have been determined by ammonia adsorption calorimetry. Ammonia is irreversibly absorbed, with the formation of the corresponding ammonium salts. An increase in the number of protons in Keggin heteropolyanions decreases the acidic strength. 

Heteropolyacids are polyoxometalates incorporating anions (heteropolyanions) having metal-oxygen octahedra as the basic structural units. Among a wide variety of heteropoly acids those belonging to the so-called Keggin series are the most important for catalysis. They include heteropolyanions where X is the central atoms (P, Si , etc.), y... 

The acid strength of Keggin-type heteropolyacids is stronger than such conventional solid acids such as Si02-Al203 and H-Y zeolites. The acid strength of crystalline heteropolyacids decreases in the series. ... 

The use of PVA-PEG beads as a support of TPA enables to retain the primary Keggin structure of the heteropolyacid, as seen through the physical-chemical characterizations. 

The development of catalysts obtained by means of heteropolyacids (HPA) and related compounds is a very important growing field. Since HPA are less corrosive and produce lower amount of wastes than conventional acid catalysts, they can be used as replacement in ecofriendly processes. Within the HPA, there is a special interest in those which present a Keggin type structure. 

Relatively high thermal structural stability the Keggin anion in heteropolyacids begins to decompose at temperatures close to 250-300°C. Salification leads to a remarkable improvement in the stability, allowing operations up to 400-450°C to be carried out. Salts can be prepared by ion-exchange in aqueous solutions of proton form of the heteropolyacid, or by direct precipitation of the insoluble salt. In some cases, such as the cesium salts of 12-molybdophosphoric or tungstophosphoric acids, the structure is stable up to 550°C. 

The Keggin-type heteropolyacid (hereafter abbreviated HPA) is a unique catalyst material because it has the dual catalytic functions of strong acidity and high oxidizing capacity [1-5]. HPA has been applied commercially as an efficient catalyst in several petrochemical processes, including the direct hydration of propene (1972) [6,7], isobutene (1984) [8] and n-butenes (1989) [9], the oxidation of metha-crolein to methacrylic acid (1982) [10], the oligomerization of tetrahydrofuran to polymeric diols (1985) [11], and the oxidation of ethene to acetic acid (1997) [12]. 

Izumi, Y., Ogawa, M., and Urabe, K. 1995. Alkali metal salts and ammonium salts of Keggin-type heteropolyacids as solid acid catalysts for liquid-phase Friedel-Crafts reactions. Appl. Catal. A Gen. 132 127-140. 

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